In cylinder washers there is a rotatable cylindrical drum, which is pervious to the washing liquid and onto the case of which pulp to be washed is fed constantly to form a mat. Washing liquid is conducted onto the top of the mat and it then passes through the mat and the case, entering the drum, from which it is removed by way of the drum end. The washed pulp is worked loose and removed from the case.
Such drum washers are also known wherein the filtrate obtained in a final washing stage-is recovered within the drum with the aid of a special collecting trough. This filtrate is then reused as washing liquid for the initial stage. Such a device is described, for example, in a brochure by the name of Rauma-Repola Pro-feed Pressure Washers, Rauma 1985.
However, it is technically difficult to bring about such collecting trough systems of the said kind, which would operate reliably.